Can aspirin reduce the risk of colorectal cancers?

May 24th's issue of The New England Journal of Medicine compares the effects of aspirin use on the relative risk of colorectal cancer.

fiogf49gjkf04

Regular use of aspirin reduces the risk of a colorectal neoplasm, but the mechanism by which aspirin affects carcinogenesis in the colon is not well understood.

Dr Andrew T. Chan and colleagues from Boston estimated cyclooxygenase-2 (COX-2) expression by immunohistochemical assay of sections from paraffin-embedded colorectal-cancer specimens from two large cohorts of participants, who provided data on aspirin use from a questionnaire every 2 years.

They applied Cox regression to a competing-risks analysis to compare the effects of aspirin use on the relative risk of colorectal cancer in relation to the expression of COX-2 in the tumor.

67% of tumors had moderate or strong COX-2 expression.

The New England Journal of Medicine

During 2,446,431 person-years of follow-up of 82,911 women and 47,363 men, the team found 636 incident colorectal cancers that were accessible for determination of COX-2 expression.

Of the tumors, 423 (67%) had moderate or strong COX-2 expression.

The effect of aspirin use differed significantly in relation to COX-2 expression (P for heterogeneity=0.02).

The age-standardized incidence rate for cancers that overexpressed COX-2 was 37 per 100,000 person-years among regular aspirin users, as compared with 56 per 100,000 person-years among those who did not use aspirin regularly.

In contrast, the rate for cancers with weak or absent COX-2 expression was 27 per 100,000 person-years among regular aspirin users, as compared with 28 per 100,000 person-years among nonregular aspirin users.

Dr Chan concluded that, "Regular use of aspirin appears to reduce the risk of colorectal cancers that overexpress COX-2 but not the risk of colorectal cancers with weak or absent expression of COX-2.